My alternative timeline America:
2) No set background. The conflict occurs sometime between 1935 and 1941. I need no rationale other than it happened.
3) Not to worry about factions, their alliegences or rationale. Concentrate on wargame forces.
4) Mix and match forces as required by each game. America is large and diverse enough so different things could have happen in different states/seaboards.
5) Not to sweat the detail. The overall look is more important!
6) Of course there are interventionist advisors and military missions from other countries!